In recent years, along with the miniaturization and integration of semiconductor integrated circuits, a semiconductor exposure device (hereinafter referred to as “exposure device”) has been required to have higher resolution. For this reason, shortening of the wavelength of light that is emitted from an exposure light source has been under development. Generally, as an exposure light source, a gas laser apparatus is used instead of a conventional mercury lamp. For example, as a gas laser apparatus for exposure, a KrF excimer laser apparatus configured to output ultraviolet laser light with a wavelength of 248 nm as well as an ArF excimer laser apparatus configured to output ultraviolet laser light with a wavelength of 193 nm may be used.
As next-generation exposure technology, immersion exposure has been put to practical use. In immersion exposure, a gap between an exposure lens in an exposure device and a wafer is filled with fluid. Since the refractive index between the exposure lens and the wafer changes, an apparent wavelength of the exposure light source is shortened. In a case where immersion exposure is performed using an ArF excimer laser apparatus as an exposure light source, a wafer is irradiated with ultraviolet light whose wavelength in water is 134 nm. This technique may be referred to as “ArF immersion exposure (or ArF immersion lithography)”.
Natural oscillation wavelengths of KrF and ArF excimer laser apparatuses are as wide as approximately 350 to 400 pm. Therefore, the constitution of a projector lens by a material that transmits ultraviolet rays such as KrF or ArF laser light may cause chromatic aberration, thus lowering resolution. Therefore, a spectrum line width of laser light that is outputted from a gas laser apparatus needs to be narrowed to the extent that chromatic aberration can be ignored. In order to narrow a spectrum line width, a line narrow module (LNM) having a line narrowing element (an etalon, a grating, or the like) may be provided in a laser resonator of a gas laser apparatus. In the following, a laser apparatus whose spectrum line width is narrowed may be referred to as a “line narrowed laser apparatus”.